In the subsea oil and gas industry, it is often desirable to perform certain fluid processing activities on the sea floor. Examples include fluid pumps (both single phase and multiphase) and compressors (both gas compressors and “wet gas” compressors). The subsea pumps and compressors are commonly driven with electric motors, which are supplied by three-phase electrical power via one or more umbilical cables from a surface facility. Especially in cases where the umbilical cable is relatively long, it is desirable to transmit the electrical power at higher voltages through the umbilical cable and use a subsea transformer to step-down a voltage suitable for use by the subsea electric motors.
High resistance grounding (HRG) is a principle that is well known and has been used in medium voltage distribution transformer systems. The purpose of the HRG is two-fold: (1) to clamp the otherwise isolated neutral point of the transformer to ground; and (2) limit possible ground fault current to a low and well defined level. In normal operation, the vector sum of the capacitive currents between the three live symmetrical phases will be zero, and no current will flow in the HRG from the transformer neutral point. With an earth fault present in one of the phases, the two healthy phases will have the correct line voltage values relative to each other both in magnitude and in phase, although they will be shifted in voltage.
In land-based medium voltage distribution systems, an HRG system is commonly arranged as an air-cooled device contained in either a separate cabinet or as free standing resistors mounted on insulators in an open arrangement in a high voltage room. In some cases, liquid neutral resistors are used in topside systems. In subsea installations, the HRG unit has been provided by a solid resistive element located in a separate compartment from the main transformer windings. FIGS. 6 and 7 are schematic diagrams illustrating aspects of subsea transformers with known HRG protection techniques.